What could this be ?

Either mechanical damage from rubbing or aftermath of heavy feeding, by look reminds more potassium toxicity, than deficiency.

It does look like K-deficiency, but symptoms alone are never enough to be 100% confident of the cause. So it's essentially a soilless grow... which helps diagnose.. If K ppm in you rmix is 180+ or thereabouts, I'l look into other causes. Maybe another nutritional element is inhibiting what should be enough K or maybe pH has drifted a bit? K is not the first element that would show issues with pH, so i doubt that is the case. 2-2,2EC is awfully high for late flower. Assuming it is well-balance, may want to knock it down to 1.5EC or lower range. Caclulating ppm from the gauranteed analysis lables are more accurate than a TDS pen that converts electrical conductivity to ppm using 1 of several factors depending on the manufacturer, which tells you the ppm readings on those TDS pens are fucking useless, lol. Plus, doesnt tell you ppm of individual elements of your fertilizer only an overall level, which is only partially useful. There are apps that do the math for you. can even calculate dosage to hit target ppms etc.. growbuddy or hydrobuddy? there's a www website too you can use from desktops/laptops, too. Keeping tack of this stuff makes diagnosing and recognizing problems a simple and nearly 100% accurate. Fertilization is about the entire time that plant is in a pot... problems arise over time that let you know if the balance is off - relative to life stage. You do need about 25-33% less N in flower, but otherwise levels are best kept in a similar range until evidence based results say otherwise, and i've looked pretty hard for such evidence and yet to find it.. just a bunch of repeated bro-science and false confidence in anecdotes.

phosphorous deficiency will cause the leaves to do this. sometimes it can be so pronounced that the leaves look bleached they are so white. it could be a ph issue if youre giving the correct supplements at the

I think it's a potassium deficiency. Now in the flowering stage, your plant needs a lot of it. But phosphorus is also important. Early symptoms: Yellow spots and discoloration on the leaf tips and edges, which eventually turn brown. Advanced symptoms: The edges of the leaves may appear burnt or necrotic. The plant shows weak growth, thin stems, and increased susceptibility to diseases. Additional notes: Potassium is more mobile than other nutrients, and deficiencies often occur in the middle to upper leaves. The pH level can rise to 6.5 during flowering, and you're good with the EC level.